Crud and Galvanic Corrosion Summary

Corrosion DOE-HDBK-1015/1-93 CRUD AND GALVANIC CORROSION Rev. 0 CH-02 Page 25 Crud and Galvanic Corrosion Summary Crud is corrosion products in the form of finely divided, insoluble oxide particles suspended in the reactor coolant or loosely adhered to metal surfaces or activated corrosion and wear products. Scale is the deposition on the surfaces of the piping from the formation of insoluble compounds from normally soluble salts. Most common are calcium or magnesium carbonates. Galvanic corrosion is the corrosion that results when two dissimilar metals with different potentials are placed in electrical contact in an electrolyte. The problems of crud in reactor plants are: Fouling of coolant flow paths Fouling of heat transfer surfaces High general background (ambient) radiation levels Radiation hot spots Radioactive waste disposal Galvanic corrosion can also be limited by: 1) using only metals that are close on the activity series (discussed in the chapter on Corrosion Theory), 2) electrical insulation of dissimilar metals, and 3) using poorly-conducting electrolytes (very pure water). The relative surface areas of the two metals are also important. A much larger surface area of the non-active metal, compared to the active metal, will accelerate the attack. It has been determined that the relative surface area is the determining factor in the corrosion rates. The required electrical current for galvanic corrosion will be stopped if the dissimilar metals are: separated by a non-conducting junction, separated from a conductive environment, and located in a poorly conducting electrolyte (pure water). Summary The important information of this chapter is summarized below.